Polar-like Codes and Asymptotic Tradeoff among Block Length, Code Rate, and Error Probability

Abstract

A general framework is proposed that includes polar codes over arbitrary channels with arbitrary kernels. The asymptotic tradeoff among block length $N$, code rate $R$, and error probability $P$ is analyzed. Given a tradeoff between $N,P$ and a tradeoff between $N,R$, we return an interpolating tradeoff among $N,R,P$ (Theorem 5). Quantitatively, if $P=\exp (-N^{{\beta }^{\ast }})$ is possible for some ${\beta }^{\ast }$ and if $R=\Capacity-N^{1/μ^*}$ is possible for some $1/{\mu }^{\ast }$, then $(P,R)=(\exp(-N^{β'}),\Capacity-N^{-1/μ'})$ is possible for some pair $({\beta }^{\prime },1/{\mu }^{\prime })$ determined by ${\beta }^{\ast }$, $1/{\mu }^{\ast }$, and auxiliary information. In fancy words, an error exponent regime tradeoff plus a scaling exponent regime tradeoff implies a moderate deviations regime tradeoff. The current world records are: [arXiv:1304.4321][arXiv:1501.02444][arXiv:1806.02405] analyzing Arıkan's codes over BEC; [arXiv:1706.02458] analyzing Arıkan's codes over AWGN; and [arXiv:1802.02718][arXiv:1810.04298] analyzing general codes over general channels. An attempt is made to generalize all at once (Section IX). As a corollary, a grafted variant of polar coding almost catches up the code rate and error probability of random codes with complexity slightly larger than $N\log N$ over BEC. In particular, $(P,R)=(\exp(-N^{.33}),\Capacity-N^{-.33})$ is possible (Corollary 10). In fact, all points in this triangle are possible $({\beta }^{\prime },1/{\mu }^{\prime })$-pairs. $$ \require{enclose} \def\r{\phantom{\Rule{4em}{1em}{1em}}} \enclose{}\r^\llap{(0,1/2)}_\llap{(0,0)} \enclose{left,bottom,downdiagonalstrike}\r_\rlap{(1,0)} \enclose{}\r $$

25 pages, 106 figures

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Cited by in corpus (1)

• Complexity and Second Moment of the Mathematical Theory of Communication